(1) Field of the Invention
The invention relates to processes for the manufacture of semiconductor devices and more particularly to processes for forming contacts/vias for interconnection wiring in low-dielectric constant insulators.
(2) Description of Prior Art and Background to the Invention
Complimentary metal oxide semiconductor(CMOS) field effect transistor(FET) technology involves the formation n-channel FETs (NMOS) and p-channel FETs(PMOS) in combination to form low current, high performance integrated circuits. The complimentary use of NMOS and PMOS devices, typically in the form of a basic inverter device, allows a considerable increase of circuit density of circuit elements by reduction of heat generation. The increase in device density accompanied by the shrinkage of device size has resulted in improved circuit performance and reliability as well as reduced cost. For these reasons CMOS integrated circuits have found widespread use, particularly in digital applications.
The basic MOSFET, whether it be NMOS or PMOS is typically formed by a self-aligned polysilicon gate process. An region of active silicon region surface for the device is defined on a silicon wafer by an opening surrounded by field oxide isolation(FOX). A gate oxide is then grown on the exposed silicon regions and a polysilicon gate electrode is patterned over the gate oxide. Source and drain regions are next formed in the active region, typically by ion implantation. The device is completed by depositing an insulative layer over the wafer and forming contacts to the source/drain regions and to the gate electrode through openings in the insulative layer. Successive levels of interconnection wiring are then formed over the device separated by dielectric layers. The wiring levels are patterned in conductive layers by photolithography and interconnected through vias in the dielectric layers.
With the introduction to DUV (deep ultraviolet) photolithography into the manufacturing of sub quarter micron integrated circuits, it was necessary to develop a new class of photoresist material which are sensitive to shorter wavelengths. DUV photolithography uses radiation at wavelengths at 248 nm. and 193 nm. derived from KrF and KrAr excimer lasers respectively. The photoresist materials used in DUV photolithography are acid catalyzed and are sensitive to the presence of trace levels of alkaline chemicals.
When these resist materials are used to pattern reflective conductive layers such as polysilicon, metal silicides, and metals such as aluminum and copper, an ARL (anti reflective layer) must be applied over the conductive layer, beneath the photoresist, in order to prevent exposure of photoresist at pattern edges by incident radiation from the reflective surface. The ARL is frequently referred to as an ARC (anti-reflective coating). Exposure by these unwanted reflections causes loss of definition at the pattern edges.
A material which has been used as an effective ARL for use with DUV photolithography because of it's high absorptive index at DUV wavelengths is silicon oxynitride (SiON, sometimes written as SiOxNy). SiON can be optimized with regard to it's anti reflection properties by varying it's composition. A variation of silicon oxynitride, having optical properties suitable for use with DUV photolithography is silicon oxime (Si(1−x+y+z)NxOy:H2, reported by Foote, et. al., (U.S. Pat. No. 6,365,320 B1). A problem encountered by the use of oxynitride ARCs under acidic photoresists involves an interaction between the acidic resist and the basic ammoniacal or amido functional groups on the SiON surface. These groups react with the lower photoresist interface to cause a sensitivity aberration at the base of the resist layer. This is commonly referred to as a footing problem.
Foote, et. al. overcomes the footing problem by forming a thin SiO2 barrier layer between the ARL and the DUV photoresist. However, this introduces an additional processing step. It therefore became desirable to eliminate nitrogen in ARLs used with DUV photoresist. This led to development of a nitrogen-free ARL (NFARL) for DUV photolithography. Silicon oxycarbide (SiOC) was found by Lee, et. al., (U.S. Pat. No. 6,376,392, B1) not only to be an effective ARL for DUV photolithography but also, eliminated the footing problem by virtue of its being nitrogen free. Lee, et. al. forms the silicon oxycarbide film using silane and methyl containing silane precursors. Silicon oxycarbide, formed by PECVD (plasma enhanced chemical vapor deposition) using an oxygen containing species, preferably N2O, and a methyl containing silane as precursors was reported earlier by Loboda, et. al. (U.S. Pat. No. 6,159,871). It was promoted by Loboda, et. al. as a low dielectric constant film but not as an ARL. However, earlier yet, Forbes, et. al., (U.S. Pat. No. 5,926,740) cited an amorphous silicon oxycarbide ARL formed either by high temperature pyrolysis of silicone resins, or by PECVD from silane, methane, and nitrous oxide precursors.
The silicon oxycarbide NFARL resolves the DUV photolithography problems when used to pattern reflective conductive layers such as polysilicon, metal silicides, and metals such as aluminum and copper. However, when used as an ARL in the patterning of vias and damascene openings in insulative layers such as silicon oxide or low-k dielectric materials, the high carbon content of the silicon oxycarbide NFARL can cause excessive polymer formation during the dielectric etch. Here etching ceases when the opening becomes pinched off with polymer before endpoint is reached.
Excessive polymer formation during etching, causes a progressive narrowing of the openings during the plasma etching and, in some instances, a pinching off of the etching before the opening is complete. The very small, high aspect ratio contact/via openings in the present sub-tenth micron technology are particularly prone to this problem. The excessive carbon is liberated into the etching plasma by the carbon-containing ARL. In conventional plasma etching of openings in non-carbonaceous dielectric layers, a steady state carbon production is beneficial to obtain vertical sidewalls in the openings. The carbon is released from the photoresist and just enough is deposited onto the sidewalls to protect them from isotropic etching by neutral species in the plasma. It is relatively easy to adjust the plasma parameters and gas flow rates to obtain a desirable steady state condition. However, when the carbon rich ARL also releases considerable carbon into the plasma, the build up of polymer along the sidewalls becomes excessive and the opening into which the directional plasma attacks the base of the opening becomes narrower and finally pinches off.
The problem is illustrated by FIG. 1a where a cross section of wafer substrate 116 with a partial opening 124 for a contact/via in a low-k inorganic dielectric layer 118 is shown. The region 116 could be either the initial bare wafer or the top layer of multiple laminar layers on the wafer which may comprise an etch stop layer or a patterned wiring layer. An excessive rate of polymer 126 formation on the opening walls has caused the opening to progressively narrow and finally pinch-off. The opening 124 was intended to have vertical sidewalls and terminate on the layer 116. A silicon oxycarbide ARL 120 with overlying DUV photoresist 122 were patterned to define the opening 124 and act as a mask for etching a high aspect ratio via in the low-k inorganic layer 118.
The present invention is geared towards reducing the release of carbon species into the etching plasma from the edge of a silicon oxycarbide ARL. While, the non-reflective thin film dielectric materials generally do not reflect incident light back through the photoresist, reflective layers in levels beneath the insulators generate significant reflections to cause aberration of the photoresist. Thus an ARL under the photoresist is still required. It is therefore desirable to have an NFARL material which has all the beneficial qualities of silicon oxycarbide with respect to DUV photolithography, while at the same time, being deficient of carbon, particularly on surfaces where the ARL is exposed during the plasma etching of small openings in the subjacent dielectric. The present invention provides a method for forming a silicon oxycarbide ARL having reduced carbon content, accomplished by replacing Si—C bonds with Si—H bonds.
Gates, et. al. U.S. Patent Application Publication 2003/0134495, cites a hydrogenated silicon carbide ARL wherein a minor amount (1–10 atomic percent) of oxygen may be added. No reason for the oxygen addition is cited. The cited ARL contains between 20 and 40 atomic % carbon. The present inventors find that, in order to reduce polymer production during plasma etching of small openings, the carbon content of the ARL must be less than about 10 atomic %, and, more preferably, less than about 5 atomic %. At the same time, in order to maintain the beneficial qualities of a silicon oxycarbide ARL, the oxygen content, as determined by the present inventors, should be greater than about 20 atomic %.
In order to further improve circuit performance, a number of low dielectric constant (low-k) materials have been developed and incorporated into the dielectric layers of modem integrated circuits. These materials provide a lower capacitance than conventional silicon oxide and consequently, an increase in circuit speed. A first category of low-k materials consists of polymers which rely on porosity and open structure for dielectric constant reduction. Examples of inorganic low-k dielectric materials which have been implemented as ILD (inter level dielectric) layers include the SOGs (spin-on-glasses) and porous silica based materials such as siloxanes, aerogels and xerogels. The porous silica materials have been developed, notably by Texas Instruments Inc. and incorporated into dual damascene processes to obtain dielectric layers with dielectric constants as low as 1.3. This is to be compared with a dielectric constant of about 4 for conventional silicon oxide.
Organic low-k materials such as fluorinated polyarylene ethers, for example FLARE™ (FLuorinated ARylene Ether provided by Allied Signal Inc., 101 Columbia Road, P.O. Box 4000, Morristown, N.J. 07962) and PAE II™ or Lo-KT™ 2000 (Poly Arylene Ether provided by the Schumacher Chemical Company which is a unit of Air Products and Chemicals, Inc., 7201 Hamilton Boulevard, Allentown, Pa. 18195–1501), have been added to the growing family of low-k and ultra low-k dielectric materials. These totally organic, non silicaceous, materials are seeing an increased usage in semiconductor processing technology not only because of their favorable dielectric characteristics, but also because of ease of application.
Quasi-organic low-k materials such as hydrosilsesquioxanes (HSQ) and fluorinated silica glass (FSG), low carbon polysilsesquioxanes, and organosilicate glasses (OSGs), for example Black Diamond™, from Applied Materials Corporation of Santa Clara Calif., have dielectric constants as low as 2.6–2.8. The low carbon polysilsesquioxanes are low density polysilicate glasses which contain alkyl or aryl groups in place of hydrogen. Procedures for application and curing of methyl silsesquioxane low-k polymer films are cited in Chua, et. al. U.S. Pat. No. 6,121,130.
While the organic low-k materials are typically etched in plasmas containing oxygen, the quasi organic and low carbon content organosilicate glasses may be etched with fluorocarbon plasmas with no oxygen content. Li, et. al., U.S. Pat. No. 6,168,726 B1, cites a number of oxygen free fluorocarbon etchants for Black Diamond™ and for HSQ. Lin, et. Al., U.S. Pat. No. 6,372,661 B1 cites the deposition of in Black Diamond™ films and formation of damascene openings therein but does not address problems of excessive polymer formation during plasma etching of narrow contact/via openings.
It is observed by the present inventors, that when sub-tenth micron size contact or via openings, especially, high aspect ratio openings, are etched with fluorocarbon plasmas in low carbon content organosilicate glasses and alkyl polysilsesquioxane low-k layers, the carbon released from these materials causes excessive and undesirable polymer formation which leads to narrowing and, in worst cases, pinching off of the openings, as illustrated in FIG. 1, regardless of the composition of the ARL. The present invention provides a method to overcome this problem as well as that regarding the aforementioned ARL.